Specificly I would like to track my grub.conf (/boot/grub/grub.conf) and some oracle files (i.e. /db/app/oracle/product/10.2.0/db_1/network/admin/tnsnames.ora).

I attempted using links; however etckeeper/git only tracks where the link points to, not the actual contents. And I can not create hard links as the files are on another volume.

I know I could setup another GIT repository but I would rather have it all within etckeeper.

Update

Based on nealmcb's answer I came up with the following script:

#!/bin/sh
set -e
# Based on nealmcb's idea/script from http://serverfault.com/questions/211425/
# If you want other configuration data or files on the system also
# opportunistically tracked via etckeeper, use this script to copy them in.
# If there is a hook of some sort available related to the files
# you're mirroring, you can call etckeeper directly and track them
# proactively, rather than just opportunistically here.
MIRROR_ROOT=/etc/etckeeper.mirror.d
echo "etckeeper: mirroring outside files to $MIRROR_ROOT/:"
mirror_dir() {
LOCAL_PATH=$1
echo " $LOCAL_PATH"
mkdir -p $MIRROR_ROOT/$LOCAL_PATH
rsync -a $LOCAL_PATH/ $MIRROR_ROOT/$LOCAL_PATH
}
mirror_dir "/boot/grub"
mirror_dir "/root"

To add or remove a path you simply add or remove the mirror_dir call at the bottom.

I also wanted to track changes made by update-grub in /boot, so
I put the code below in /etc/etckeeper/commit.d/20mirror-outside-files

This way any time etckeeper is called for other reasons (when I install software, sometimes nightly, etc), it will grab and track revisions in the latest grub configuration.

I invented the convention to put this stuff under /etc/Mirror/path-to-outside-file, e.g. /etc/Mirror/boot/grub/grub.cfg but if anyone has precedent for another such convention, I'd love to hear about it.

#!/bin/sh
set -e
# If you want other configuration data or files on the system also
# opportunistically tracked via etckeeper, use this script to copy them in.
# If there is a hook of some sort available related to the files
# you're mirroring, you can call etckeeper directly and track them
# proactively, rather than just opportunistically here.
echo etckeeper: mirroring outside files
mkdir -p /etc/Mirror/boot/grub
cp -p /boot/grub/grub.cfg /etc/Mirror/boot/grub

Update:

Note that for some reason etckeeper doesn't run this when you do an apt-get remove or purge, e.g. to delete an old kernel. Odd.... But you can run sudo etckeeper commit manually in that case, or after a manual update-grub.

While the reverse link is a good idea, and I use it in some cases there are cases where it won't work. Specifically don't try the reverse link for your grub.conf or else you won't be able to boot.
–
ZoredacheDec 10 '10 at 23:05

@Zoredache - Exactly. However the reverse link would probably work for my Oracle files. And and @pjz's copy method would probably work for anything on boot (grub + kernel conf). I will try this next week.
–
ErebusBatDec 11 '10 at 18:17